He pledged to make clear to Americans the reasoning behind his new strategy and its costs.

"The American people will have a lot of clarity about what we're doing, how we're going to succeed, how much this thing is going to cost, what kind of burden does this place on our young men and women in uniform and, most importantly, what's the end game on this thing," he said.

In the wake of the political upheaval that surrounded President Karzai's disputed re-election, Mr Obama also expressed concerns about the Afghan government's ability to be an "effective partner".

"I'm less concerned about any individual than I am with a government as a whole that is having difficulty providing basic services to its people in a way that confers legitimacy on them," Mr Obama said.

"We also have to make sure that we've got an effective partner in Afghanistan. And that's something that we are examining very closely and presenting some very clear benchmarks for the Afghan government."

An ABC News/Washington Post poll suggests 44% of US adults believe the Afghan war is worth fighting, the lowest proportion since polls began in 2007.

The US currently has about 68,000 troops in Afghanistan, contributing to a coalition force of more than 100,000.

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